This invention relates to wind turbines and wind turbine blades, and to the reduction in reflectivity of such turbine blades to electromagnetic radiation.
Renewable energy targets rely heavily on wind power, both onshore and offshore. However, a significant percentage of all planning applications for new wind farms are rejected on the grounds that they will interfere with civil airport radars. The use of radar absorbing materials (RAMs) for wind turbines has previously been proposed to reduce the effect on ATC (Air Traffic Control) radar and ADR (Air Defence Radar) by reducing the RCS (Radar Cross Section). Wind turbine blades are highly specified to meet stringent performance criteria however, and the introduction of RAM to the blades ideally results in little or no modification of the structural properties and/or weight of the blade. This presents a significant engineering challenge.
The present applicant has produced a report entitled “Design and manufacture of radar absorbing wind turbine blades—final report” as part of a DTI funded study. In this report the glass reinforced epoxy (GRE) and GRE/foam sandwich construction of certain turbine blades is noted, and it is suggested that a Jaumann type absorber could be used by including a resistive layer at a specific spacing from a perfect electrical conductor (PEC). The outermost GRE skin can be modified using a lossy impedance layer, and a reflector included on the inside surface of the sandwich construction. It is stated that impedance layers can be made from glass fibre tissues impregnated with a small amount of chopped carbon fibres. A carbon fibre cloth layer can act as the PEC, inserted into the rear of the GRP skin.
It is an object of the present invention to provide improved wind turbine blades and associated methods.